Electrolysis cell current efficiency with oxygen-containing gases

ABSTRACT

A FUSED SALT ELECTROLYSIS CELL BATH IS TREATED WITH AN OXYGEN-CONTAINING GAS, SUCH AS AIR, WHEREIN THE TREATING IS EFFECTED OUTSIDE THE ELECTROLYSIS CELL.

United States Patent M 3,560,353 ELECTROLYSIS CELL CURRENT EFFICIENCY WITH OXYGEN-CONTAINING GASES Rudolf E. Svadlenak, Lewiston, N.Y., assignor to E. I.

du Pont de Nemours and Company, Wilmington, Del.,

a corporation of Delaware No Drawing. Filed Sept. 26, 1967, Ser. No. 670,799

Int. Cl. C22d 3/06 US. Cl. 204-68 3 Claims ABSTRACT OF THE DISCLOSURE A fused salt electrolysis cell bath is treated with an oxygen-containing gas, such as air, wherein the treating is effected outside the electrolysis cell.

BACKGROUND OF THE INVENTION In the operation of alkali metal fused salt cells, a fused salt mixture is electrolyzed to produce alkali metal at the cathode and halogen gas at the anode. The anode is conventionally a cylindrical graphite or carbon anode surrounded by an annular metallic cathode. A porous diaphragm is provided in the anode-cathode annular space to assist in the separation of the products of electrolysis.

Metallic sodium is produced generally from a molten mixture of the chlorides of calcium and sodium in electrolytic cells of the Downs type (US. Pat. No. 1,501,756) or in modifications of these cells. These cells are characterized by having one or more bottom mounted vertically aligned cylindrical graphite anodes each of which is projected upwardly into a separate cylindrical opening within a unitary cathode assembly. The sodium is produced at the cathode surfaces and the chlorine at the anode surfaces. Due to the difference in densities between these products, and that of the molten bath, the products rise to the surface of the bath and are then collected.

The aforementioned cells of prior practice have given good results, but unfortunately cells of this type rarely ever achieve current efficiencies greater than 80-85% (cf. Sodium, A.C.S. monograph 133, p. 31, M. Sittig, Reinhold Publishing Corporation, 1956). The current efficiency of these cells increases as the operating temperature is reduced. However, crust formation or any bath solidification interferes seriously with economical operation so that satisfactory results can be secured only when the baths are maintained at temperatures above the melting point.

Alternate salt mixtures for use as electrolyte baths have been described in the prior art and, although some of these mixtures have shown improved current efficiencies, this advantage has been offset by high material costs or production of sodium whose quality does not meet present high purity requirements and is not readily purified. Grabau, US. Pat. 464,097 (Dec. 1, 1891), disclosed a ternary mixture consisting of sodium chloride, another alkali metal halide and alkaline earth halides. This mixture was stated to have a current efficiency of 95% but yielded sodium containing another alkali metal as an impurity. With the preferred other alkali metal salt, potassium chloride, the sodium was stated to contain 3% potassium. Seward et al., US. Pat. 841,724 (Jan. 22, 1907), described a mixed salt bath containing sodium chloride, sodium fluoride and an alkali earth chloride. This bath gave sodium free of other alkali metals but its current efficiency was not disclosed. However, neither of these baths appeared to have found commercial acceptance. More recently, Cathcart et al., US. Pat. No. 2,850,442, have devised a mixture consisting of sodium Patented Feb. 2, 1971 chloride, barium chloride and strontium chloride. This electrolyte yields sodium containing less than 0.1% impurities and shows current efficiencies of 89% which can be raised to -95% by addition of 1-2% sodium fluoride. However, this mixture is expensive because of its strontium compound in that the original investment and maintenance are of a relatively high order. Wood, US. Pat. No. 2,876,181, obtains high current efficiency with a lithium chloride-sodium chloride electrolyte but the sodium is stated to contain 4% lithium under exemplary operating conditions and a lithium electrolyte is expensive.

It is apparent that a tremendous effort has been exerted over the past years to achieve greater current efficiency. In view of the magnitude of the sodium industry coupled with the importance of sodium as a chemical intermediate and the rising power costs, the development of a relatively low cost, readily operable bath of improved current efficiency is of outstanding industrial importance.

SUMMARY OF THE INVENTION This invention relates to a novel process for improving the current efficiency of an alkali metal fused salt electroylsis cell comprising treating the molten cell bath with an oxygen-containing gas, wherein the treating is effected outside the electrolysis cell.

It has been discovered that this oxidation treatment increases the current efficiency of a used fused salt electrolysis cell in a cheap, eflicient and commercially applicable manner.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The treatment is carried out by passing an oxygencontaining gas into and/or through a molten cell bath by any conventional means such as by sparging. However, this treatment with an oxygen-containing gas is effected outside the electrolysis cell. In this manner, the oxygencontaining gas does not have any direct effect on the electrolysis reaction and sodium collection which are being carried out inside the cell itself. If the treatment with the oxygen-containing gas is effected inside the cell, the oxygen would react with the sodium being collected and produce hazardous working conditions (e.g. fires). Therefore, it is very important to effect the treatment outside of the electrolysis cell. Additionally, this process makes it possible for used electrolysis cell baths to be regenerated outside of the cell bath before further use in the actual electrolysis cell.

In normal commercial operations, a sodium cell may be operated for a period of several months before the current efficiency decreases significantly. At this point a portion or all of the used cell bath may be removed from the electrolysis cell and placed in a separate container. The cell bath in the container is maintained in a molten state and the oxygen-containing gas is then passed into this molten cell bath. The preferred method involves sparging the molten cell bath for any desirable time. Generally, the cell bath current efficiency is increased within a short period of time, depending upon the purity and condition of the used cell bath.

The oxygen-containing gas may be any which contains some oxygen. Of course, gases which contain significantly large amounts of oxygen are preferred since the oxygen is the gas which oxidizes the cell bath to improve current efficiency. The preferred oxygen-containing gases are air and elemental oxygen. Also within the scope of the invention are mixtures of inert gases such as argon, krypton or helium and air, ozone or oxygen.

The amount of oxygen-containing gas which is passed into the cell bath may vary according to the condition of the cell bath, the desired degree of oxidation and the speed of oxidation. Generally, an amount of oxygen-containing gas added to the cell bath can range from about 0.01100-% by volume of the cell bath. The preferred range of 0.550% by volume has been found to produce the desired degree of oxidation and improved current efiiciency.

The reasons for a current efiiciency recovery from the oxidation treatment are not fully understood. While this invention is not intended to be based upon any particular theory, it is speculated that poor current efficiency may be due to the presence of small particulate electronic conductors in the bath which serve to transfer metallic sodium to the anolyte by a concentration cell mechanism. At the anolyte the sodium is destroyed by the direct combination with chlorine. The addition of an oxygen-containing gas serves to change the particulate electronic conductor to an inert oxide. Thus, current efficiency lost by a concentration cell mechanism cannot take place.

The following examples are illustrative of the practice of this invention. All electrolyte compositions are reported as percent by weight.

EXAMPLE 1 A substantially conventional Downs cell was charged with an electrolyte containing about 26% sodium chloride, 24% calcium chloride and a remainder of barium chloride. This bath had a melting point of about 560 C. and was operated with a direct current for several months at an average temperature of about 605 C. This cell was equipped with a wire mesh diaphragm between anode and cathode and the spacing between these electrodes was 1.5 inches. Electrolysis was carried out at about 38,000 amps. and at a cell voltage of about 7 volts.

A portion of the cell bath was transferred to a laboratory compartmental cell where the current efiiciency was found to be 73%. Another portion of the above-described Downs cell bath was transferred to a beaker. While the cell bath was maintained in the molten state in the beaker, air was sparged into the molten cell bath for 35 minutes. The current efficiency of this treated cell bath was measured and found to be 91%. This 18% increase in current efliciency is attributed to the treatment with air.

Several cell bath samples were taken from conventional Downs cells which were operating in a plant on a commercial scale. The cell baths contained similar electrolyte compositions as described above and were operated for various periods of time ranging from 2-12 months. In all 4 cases the current efficiency was measured in a laboratory cell before and after the treatment with a gas. Results are reported in the following table.

It is pointed out that Example 4 is not within the scope of this invention since it is not an oxygen-containing gas. This example clearly demonstrates that inert gas does not achieve the desired results since the cell bath current etficiency is not improved from sparging with nitrogen.

Since it is obvious that many changes and modifications can be made in the above-described details without departing from the nature and spirit of the invention, it is to be understood that the invention is not to be limited to said details except as set forth in the appended claims.

What is claimed is:

1. A process for improving the current efiiciency of an alkali metal fused salt electrolysis cell comprising removing at least a portion of the used alkali metal fused salt bath to a separate container, contacting the removed molten bath with an oxygen-containing gas, and returning the regenerated bath to the electrolysis cell.

2. A process in accordance with claim 1 wherein the gas is air.

3. A process in accordance with claim 1 wheren the gas is pure elemental oxygen.

References Cited UNITED STATES PATENTS 2,111,264 3/ 1938 Gilbert 204-68 2,414,831 1/1947 MCNitt 20468 3,400,060 9/ 1968 Gallinger 204246X JOHN H. MACK, Primary Examiner D. R. VALENTINE, Assistant Examiner 

